Hello there i've been lurking for a while, but registered to hear some expert opinions on the 50% haste soft cap as a protection paladin.

I'm currently raiding 10man sitting on unbuffed 36.6% haste (15,557). When in a raid group from a rogues swiftblade's cunning buff it takes me up to 50% haste. I have noticed there are other hardcore paladin tanks such as treckie for example with 44.7% haste unbuffed, no doubt he will have the melee haste raid buff as well in his set up.

I was wondering if continuing to stack haste even though it would take me over this soft cap was the way to go over some additional stamina/mastery? Would love to hear your opinions on this!

The 10% melee attack speed buff is just that: Attack speed. It does not grant actual haste at all. If it did, that would have already been factored into the math. Feel free to continue stacking haste until 50%/21250 haste rating if that's what strikes your fancy.

Do note that cutting edge progression guilds tend to favor stamina at the start so they can get used to the fights and find a good comfort zone.

Also, it's likely that you won't reach the 50% haste cap, not without needlessly gimping yourself in other areas. When the 5.4 items trickled in from the PTR, there were 2 haste trinkets that both looked quite interesting, and which would surely bring us to cap (those trinkets alone provided a quarter of the required amount of rating), but they have been overhauled, so there's only one trinket that affects our haste in 5.4, and it increases our haste by less than the Spark of Zandalar does.

The Melee Attack Speed buff is still useful for us, as it increases the number of SoI procs (and of course deals more white damage), even beyond 50%.